Method of producing monocrystalline wafers from the vaporous phase with alternative cooling and intermediate holding steps



1965 w. KRIEGLSTEIN ETA'L 3,153,423

. METHOD OF PRODUCING MONOCRYSTALLINE WAFERS FROM THE VAPOROUS PHASE WITH ALTERNATIVE COOLING AND INTERMEDIATE HOLDING STEPS Filed March 15, 1965 FIG.1

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United States Patent 3,163,423 METHOD OF PRODUCING MONOCRYSTALLINE WAFERS FRQM THE VAPGRGUS PHASE WITH ALTERNATIVE COOLING AND INTERMEDIATE HOLDING STEPS Walter Krieglstein, Rosstal, near Numberg, and Bruno Reiss, Erlangen, Germany, assignors to Siemens- Schuckertwerke Aktiengeselischaft, Berlin Siemensstadt and Erlangen, Germany, a corporation of Germany Filed Mar. 13, 1963, Ser. No. 264,805 Claims priority, appiication Germany, Mar. 15, 1962, S 78,498 Ive/12c 8 Claims. (Cl. 148-175) ous state. Thereafter a portion of the vessel is cooled at such a rate that the slope of the cooling curve is in the range from about -15 to about -140. As a result, leaf-shaped mono-crystals of the semiconductor material are grown in the reaction space. By adding doping substances to the materials in the reaction chamber, the resulting monocrystals can be given a desired type of conductance. With the aid of such methods there have been produced wafer or flake-shaped monocrystals of silicon, germanium and semiconductor compounds of the type AIIIBV Relating generally to a method of growing monocrystals by precipitation from the vaporous phase, it is an object of our invention to afford applying the same principle of growth to the production of single or multiple p-n junctions during the performance, or in immediate continuance, of the process employed for producing the substrate required for such junctions. In other words, it is an object of our invention to combine within a single continuous operation the production of a monocrystalline substrate as well as of one or more epitaxial layers on top of the substrate.

Another object of our invention is to improve crystal growing methods of the above-mentioned kind by considerably increasing the yield.

To achieve these objects, and in accordance with a feature of our invention, the method of producing monocrystalline waters or layers from the vaporous phase in a cooled zone of an evacuated reaction vessel is carried out by conducting the temperature-reduction program in the cooling zone in a non-uniform, incremental manner so that the cooling takes place rapidly in individual steps between the high temperature of commencing precipitation down to the low temperature at which the precipitation substantially terminates. The rapid cooling during each of the successive steps is understood to require a cooling rate of 8 to 50 C. per minute. Each individual cooling step is followed by a temperature holding interval. That is, each interval in which a step of temperaice ture reduction takes place is followed by an interval of generally comparable duration in which the temperature is held at approximately the magnitude reached at the end of the preceding cooling step, before the next following cooling step is commenced. As a result, there occur flake-shaped crystals which grow in consecutive layers, one on top of the other, each layer being produced in one of the respective cooling-step intervals of the temperature-reduction program.

Due to the non-uniform, incremental and rapid temperature reduction in the cooling zone of the reaction vessel, layers of any desired thickness can be precipitated during each cooling interval upon substrates previously present or produced in the reaction space. Another advantage is the fact that during the crystallization process, doping substances in vaporous form can be admixed to the reaction vapor prior to a selected cooling step with the result that a correspondingly doped layer is grown on the crystal during the next following cooling step. This aifords producing a sequence of layers Whose respective conductance values or conductance types are different. For example wafers of the n-p-n or p-n-p junction type can thus be obtained by admixing to the reaction vapor correspondingly different doping substances prior to re spective sequential cooling intervals. In this respect the invention is superior to the known methods which do not permit producing such layer sequences in a single and continuous crystal-growing operation but require the finished vapor-deposited crystals to be subjected to a subsequent diffusion or alloying process.

According to another feature of our invention, the yield of the method can be increased by raising the temperature for a short interval of time after termination of the individual temperature-reduction steps and prior to maintaining the temperature constant during the next following holding interval. The slight increase in temperature at the end of each cooling step, raising the temperature to a value considerably below that obtaining at the beginning of the preceding cooling step, has the effect that small compact crystallites are converted back to the vaporous phase. As a consequence, the reaction vapor is utilized only for growing relatively large wafers but not for spurious additional depositions too small to be used in practice. Thus the yield of the precipitation method is improved considerably.

Applicable for the method of the invention are, for example, all semiconducting elements and various semiconductor compounds. Particularly well suitable are the semiconducting elements Si and Ge and the semiconductor compounds of the A B type, such as GaP and GaAs. The method is also applicable to other substances, such as Cu or Ag, as long as the substance, whether elemental or a compound reacts with oxygen, sulphur or halogen, for example iodine or bromine, and is then available in form of a vaporous compound at sufficiently high temperature. When the temperature of such a vaporous compound is being reduced, the compound becomes dissociated and the liberated starting element or starting compound is precipitated.

Halogens have been found particularly well suitable for converting such elements or compounds to the vaporous phase, especially with respect to semiconducting elements of the fourth main group of the periodic system and semiconductor A B compound formed of respective elements from the third and fifth groups of the periodic system. Halogens react at relatively low temperatures with the above-mentioned elements which may have different valencies depending upon temperature. That is, there occur equilibrium reactions as exemplified by the following gallium-halogen reaction:

+ rising temp.

falling temp.

in which Ha denotes a halogen. According to the formula, gallium is dissolved when the temperature increases, and the equilibrium is then increasingly displaced toward the right, whereas a decrease in temperature causes the reverse conditions to prevail. Then an element, in this case the element from the third group, is liberated and can react with any element from the fifth group as may be present in vaporous form, thus resulting in an A B compound.

The invention will be further explained with reference to the drawing and with reference to specific examples.

On the drawing:

FIG. 1 shows schematically a device for performing the method of the invention.

FIG. 2 shows schematically another embodiment of a device for performing the method of the invention; and

FIG. 3 is a graph explanatory of the non-uniform, incremental cooling operation typical of the method according to the invention.

Illustrated in FIG. 1 is a reaction vessel 11 in form of a quartz ampule. The length of the ampule is approximately 250 mm. Its diameter is between and 40 mm., preferably about 30 mm. The reaction vessel is joined with a lateral branch pipe 12 whose lower end 13 is widened. constrictions are provided at 14 and 15 to provide fusing and sealing points. A ground and polished conical nipple 16 serves for connecting the device through a cooling trap to a mercury diffusion pump (both not illustrated). The use of the device according to FIG. 1 will be explained in conjunction with Examples 1 through 6 described further below.

FIG. 2 shows an ampule-shaped reaction vessel 21 positioned in an electric furnace 22. The vessel has a special shape particularly suitable for the production of p-n junctions. Aside from the reaction space 23 for growing crystals, the vessel is equipped with two chambers 24 and 25 for accommodating respectively different doping substances. The chambers contain respective capillaries 26 and 27 which can be destroyed by quartz blocks 12-8 and 29. When slightly tilting the quartz ampule, one of the quartz blocks will drop against the tip of the capillary and thereby open this capillary. The ends of the furnace are closed by stoppers 200 of ceramic foam material to prevent chimney action. A bore 201 in one of the stoppers 200 permits inserting a thermocouple for controlling the temperature in the interior of the furnace.

Also shown in FIG. 2 is a coordinate scale of indicia indicating on the abscissa the length L of the furnace in millimeters and on the ordinate the temperature T in C. The curve 202 denotes the temperature at different localities along the tubular interior of the furnace. By means of an external mirror (not shown) and with the aid of a second bore 201 in one of the stoppers 200, the progressing precipitation and crystalline growth in the interior of the reaction vessel can be observed.

FIG. 3 shows a graph indicative of the non-uniform cooling program to which the ampule and its contents are subjected after heating them to eifect vaporization of the contents. The abscissa in FIG. 3 indicates the cooling time in minutes. The ordinate indicates temperature T in C. The curve 31 represents the entire course of the cooling program within the incandescent range where dissociation of the vaporous compound and precipitation and crystalline growth take place in the reaction space. The cooling process comprises a plurality of steps. Each cooling step corresponds to a curve portion 32 and is followed by a short-lasting increase in temperature represented by short ascending curve portions 33. The next following curve portions 33, extending substantially parallel to the abscissa, constitute the intermediate intervals in which the temperature is kept constant.

EXAMPLE 1 Production of gallium-arsenide flakes Employed is the device according to FIG. 1. The device is cleaned with aqua regia, then rinsed with twice distilled water, dried at C. and then heated for one hour at 500 C. in high vacuum, namely at a pressure below 2-10 mm. Hg. After cooling, the device is filled with purified nitrogen. Thereafter an amount of 410 mg. gallium in form of globules is entered into the ampule 11. An amount of 475.6 mg. fi-arsenic and 1495 mg. iodine are entered into the widened portion 13 of the branch 12. The ampule has a length of 240 mm. and an inner diameter of 30 mm.

After thus filling the ampule it is evacuated, a cooling trap being inserted between the connecting nipple 16 and the mercury diffusion pump being used. The portion 13 of the lateral branch 12 is then filled with liquid nitrogen. The cooling of portion 13 is effected after elapse of about 20 seconds so that the water still contained in the iodine can condense in the cooling trap. After attaining the ultimate pressure of 2-10- mm. Hg, the device is fused fused off at 14. Thereafter, iodine and arsenic are sublimated from portion 13 into the reaction vessel proper 11. Then the reaction vessel is fused off at 15. The ampule 11 is shoved into the middle of the tubular furnace whose temperature is supervised and controlled by means of a thermocouple. The furnace and the reaction chamber are heated within two hours up to 1000" C., and this temperature is maintained for an additional period of two hours. Under these conditions, a total pressure of 4.9 atmospheres obtains in the reaction chamber at 1000 C.

For performing the cooling operation, the electric heating is switched off. The furnace with the ampule now cools down during seven minutes at a rate of 10 C. per minute, thus reducing the temperature a total amount of 70 C. During this temperature-reduction interval, the crystal flakes or wafers develop and grow. When about 930 C. are attained, the electric heating is again switched on so that a slight temperature increase of about 8 C. is produced. Only thereafter is the temperature kept constant for six minutes. This is followed by the second cooling step, but only down to 900 C. During this second step, another monocrystalline layer is precipitated upon the flakes previously grown. Thereafter the temperature is again slightly increased and then kept constant for about four minutes. No cooling steps should be performed below 800 C. because the major portion of the material is already precipitated before that temperature is reached. With increasing number of cooling steps, the duration of the cooling and temperature-holding intervals decreases accordingly. After terminated reaction, the ampule is taken out of the furnace and cooled down to room temperature. After opening the ampule, the monocrystalline GaAs wafers can be removed.

EXAMPLE 2 Production of gallium-phosphide wafers Employed is the same device as in Example 1, except that the length of the quartz ampule (FIG. 1) is mm. and its inner diameter 17 mm. The ampule is provided with 81.6 mg. gallium in form of globular granules. The branch portion 13 is filled with 60.5 mg. phosphorus and 297 mg. iodine. Fusing-off is eifected as in Example 1. Then the ampule is heated within two hours in the furnace to 1100 C. This temperature is maintained for three hours. For performing the cooling operation, the electric heating circuit is opened and the furnace is permitted to cool for three minutes down to 1070 C. Then the heating circuit is supplied with a somewhat reduced voltage for a short interval of time to increase the temperature by about 8 C. Thereafter the temperature is kept constant for three minutes. The next cooling step is then applied for reducing the temperature to 1020 C., this is followed by a slight increase in temperature and another holding interval in which the temperature is kept constant. These steps are repeated down to about 900 C. Thereafter the furnace is completely disconnected and the ampule is permitted to cool down after being taken out of the furnace.

EXAMPLE 3 Production of germanium wafers Employed is the same device as in Example 1. The quartz ampule has a volume of 170 ccm. The pre-treatment is performed as in Examples 1 and 2. The quartz ampule is supplied with 500 mg. germanium and 1780 mg. iodine and thereafter fused off at a vacuum of 10 mm. Hg. The ampule is heated in the resistance furnace for one and a half hours to a temperature of 800 C. This temperature is then maintained for about three hours before the temperature is reduced to 740 C. within two minutes by opening the electric furnace circuit. Thereafter the temperature is increased by 6 C. and then kept constant four minutes. The next following cooling step is performed down to a temperature of about 660 C. and lasts about three minutes. The then following increase in temperature is C. and the temperature then reached is kept constant for an only short interval of time. Thereafter the furnace is cooled for about two minutes at a rate of 30 C. per minute. Then the ampule is taken out of the furnace and permitted to cool to room temperature.

EXAMPLE 4 Production of silicon wafers Employed is a device according to Example 1, cleaned with aqua regia, twice distilled water and alcohol. The volume of the quartz ampule is 120 ml. The weighed-in amounts placed into the ampule are 136 mg. silicon and 1230 mg. iodine.

The ampule is fused off at a pressure of 1-10- mm. Hg. The heating of the ampule up to 1150 C. is performed within two hours. This temperature is maintained for two hours. Thereafter the ampule is cooled in the furnace Within two minutes down to 1100 C. Then the temperature is increased about 10 C. and subsequently kept constant for ten minutes. The next cooling step is effected down to 1020 C. Then the temperature is increased about 5 C. and the temperature reached is kept constant four minutes. Thereafter the ampule is taken out of the furnace and cooled to room temperature.

EXAMPLE 5 Production of doped gallium-phosphide wafers Employed is the device according to FIG. 1 as described with reference to Examples 1 and 2. The ampule 11 is provided with 106 mg. gallium and 8 mg. zinc. Placed into the branch portion 13 are 51 mg. phosphorus and 390 mg. iodine. The device is then evacuated down to a pressure of 1-10- mm. Hg and fused off at 14. Subsequently the phosphorus and the iodine are sublimated into the ampule 11 which is then fused off at 15. The ampule is subsequently heated within one hour to 1100 C. This temperature is kept constant for three hours. Then the ampule is cooled down to 1060 C. within two minutes. At the end of this interval, the temperature is briefly increased about 10 C. and then kept constant for three minutes before the next cooling step is commenced. The further processing is as described above with reference to Example 2. After about twenty minutes the ampule is taken out of the furnace and cooled to room temperature. After opening the ampule, the p-doped Wafers can be removed.

6 EXAMPLE 6 Production of doped gallium-arsenide wafers The quartz ampule used is in accordance with FIG. 1 and is pre-treated as described in Example 1. The ampule has a volume of 118 ml. The weighed-in amounts are 284 mg. gallium, 330 mg. arsenic, 1035 mg. iodine and 10 mg. zinc. The further processing is analogous to Example 1. After 1000 C. are reached and are kept constant for two hours, the electric furnace circuit is interrupted and thereby the ampule cooled down to 930 C. This is followed by a slight increase in temperature by about 8 C., and the temperature then reached is kept constant. The further temperature program is in accordance with the Example 1. After termination of the reaction, the ampule is removed from the furnace, cooled down to room temperature and opened so that the p-doped wafers can be removed.

EXAMPLE 7 Production of p-n-p gallium-arsenz'de wafers Employed is the device according to FIG. 2. The quartz objects are cleaned with aqua regia twice distilled water and alcohol. Thereafter the device is dried in a drying cabinet at C., then heated at 500 C. and a pressure of 10- mm. Hg.

The inner portion 23 of the quartz ampule 21 is so prepared that the one side at 26 is already available as a capillary. Supplied into the inner portion 23 are mg. gallium, mg. arsenic, 600 mg. iodine, together with 5 mg. sulphur to act as dopant. Thereafter the ampule is fused off at a pressure of 5-10 mm. Hg. The volume of the ampule is 70 ml. Thereafter the portion 24 is fused to portion 23 and provided with 20 mg. zinc, as well as with the quartz block 28. This portion 24 is to be kept as small as feasible. It is fused off at a pressure of 1-10* mm. Hg. The space 25 is not needed for the production of p-n-p junctions.

The entire ampule, thus prepared, is heated within two and one half hours up to 1000 C. This temperature is maintained for three hours. Then the ampule is cooled for about five minutes at a rate of 10 C. per minute. This is followed by a short temperature increase of about 8 C. The resulting temperature is kept constant four minutes. During this operation, there result n-doped gallium arsenide wafers. At the end of the cons-tanttemperature interval, the quartz block 28 is flung against the capillary 26 and destroys it so that zinc vapor can penetrate into the portion 23 of the ampule 21. Thereafter the heating performance is interrupted for three minutes so that the ampule cools down to about 920 C. During this cooling interval a zinc-doped monocrystalline layer is grown on both sides of the n-doped wafer previously produced. As a result, a p-n-p junction is obtained. Since the major portion of gallium arsenide is already separated, the ampule is taken out of the furnace after :a short period of time and cooled down to room temperature. After opening the ampule, the p-n-p Wafers can be removed.

EXAMPLE 8 Production of p-n-p doped germanium wafers Employed is the device according to FIG. 2 in the manner described in Example 7. The total volume of the quartz ampule is 85 ml. The space 23 of the ampule is filled with 250 mg. germanium, 900 mg. iodine as well with 5 mg. sulphur to serve as dopan-t. The amount of material placed into space 24 is 20 mg. zinc. After evacuating the ampule and fusing it on at a pressure of 1-10- mm. Hg, the ampule is heated up to 800 C. This temperature is maintained for one hour. Then the ampule in the furnace is cooled Within two minutes down to 740 C. Subsequently the temperature is increased 8 C. and kept constant for four minutes. In this interval there are formed sulphur-doped wafers 7 of germanium. At the end of the constant-temperature interval, a short impact applied to the ampule causes the quartz block 28 to destroy the capillary 26 so that zinc vapor enters into the space 23. During the next following cooling of the ampule down to 680 C. within two minutes, 'a Zn-doped layer is grown on each side of the sulphur-doped wafer, thus forming a p-n-p junction. After about minutes the stepwise cooling is discontinued and the ampule is removed from the furnace and cooled down to room temperature.

EXAMPLE 9 Production of n-p-n-p-n gallium-arsenide wafers Used is an ampule according to FIG. 2. The inner portion 23 is so prepared that the side at 26 is already designed as a capillary. 109 mg. gallium, 127 mg. arsenic, 400 mg. iodine and 4 mg. sulphur are placed into the ampule, and the ampule is fused off to form a capillary at a pressure of 1-10- mm. Hg. The total volume of the quartz ampule is 45 ml. Thereafter the portion 24. is fused to the portion 23 and provided with the quartz block 28 and with 15 mg. zinc. The portion 24 is to be kept as small as feasible. It is fused off at a pressure of 1'10 mm. Hg. Portion 25 is provided with the quartz block 29 and with mg. sulphur, and is then fused to the portion 23. The quartz ampule thus prepared is heated in the resistance furnace up to 1000 C. within two hours and is thereafter kept at this temperature for an additional period of two and one half hours. Thereafter the .ampule in the furnace is cooled within three minutes from 1000 C. to 970 C. at the cooling rate of 10 C. per minute. The temperature is thereafter briefly increased by about 6 C. and then kept constant for four minutes. During the operations just described, there are formed sulphur-doped gallium-.arsenide waters which are rather thin on account of the short duration of the cooling step. At the end of the temperature holding interval, a short impact is applied so that the capillary 26 of portion 24 is broken off, and zinc vapor can enter into the chamber 23. The excessive amount of sulphur is then compensated so that during the next following cooling interval of two minutes during which the temperatu-reis reduced down to 950 C., a layer of zinc'doped gallium arsenide is grown on both sides of the wafer. At the end of the two-minute interval, the temperature is increased by 5 C. and the resulting temperature of 955 C. is kept constant for two minutes; This is sufficient because zinc diffuses rather rapidly into gallium arsenide. At the end of the latter constan-ttemperature interval, a short impact is applied to cause quartz block 29 to destroy the capillary 27. Now sulphur vapor passes from portion into chamber 23. The next following cooling step lasts five minutes and reduces the temperature down to 900 C. This is followed by a temperature increase of 6 C. The resulting temperature is then kept constant for only two minutes. Again a thin sulphur-doped layer of gallium arsenide is grown on both sides of the water. The ampule is now removed from the furnace and cooled down to room temperature. After opening the :ampule, the n-p-n-p-n doped wafers of gallium arsenide can be removed.

We claim:

1. The method of producing monocrystalline wafers by precipitating substance from the heated gaseous phase in a cooling zone of an evacuated vessel, which comprises heating the vessel and the substance contained therein to above the vaporization temperature of the substance, and thereafter subjecting the cooling zone of the vessel to cooling at a rate of 8 to 50 C. per minute in a plurality of steps within the incandescent range, and maintaining the temperature approximately constant between the respective cooling steps for an average interval of time approximately corresponding to that of the cooling steps.

2. The method of producing monocrystalline'wafers by o precipitating substance from the heated gaseous phase in a cooling zone of an evacuated vessel, which comprises heating the vessel and the substance contained therein to above the vaporization temperature of the substance, and thereafter subjecting the cooling zone of the vessel to cooling at a rate of 8 to 50 C. per minute in a plurality of steps within the incandescent range, temporarily raising the temperature after each cooling step to a magnitude below the starting temperature of said cooling step and maintaining said temperature magnitude substantially constant before commencing the next cooling step, the average duration of the respective cooling steps being approximately the same as that of the intermediate intervals. 7

3. The method of producing crystalline semiconductor wafers with epitaxial layers, which comprises heating semiconductor substance and halogen in an evacuated vessel to reaction temperature to produce gaseous semiconductor-halogen compound in the heated vessel, thereafter subjecting a zone of the vessel to stepwise cooling at a rate of 8 to 50 C. per minute and intermediate temperature holding steps from commencing precipitation of semiconductor substance down to substantial termination of precipitation, whereby respective crystal layers are deposited upon each other during the respective cooling-step periods.

4. The method of producing monocrystalline wafers of semiconductor compound, which comprises heating the elemental constituents of the semiconductor compound and halogen in an evacuated vessel to reaction temperature to produce gaseous semiconductor-halogen compound in the heated vessel, thereafter subjecting a zone of the vessel to stepwise cooilng at a rate of 8 to 50 C. per minute and intermediate temperature holding steps from commencing precipitation of the semiconductor compound down to substantial termination of precipitation, whereby respective crystal layers are deposited upon each other during the respective cooling-step periods.

5. The method of producing crystalline semiconductor wafers with epitaxial layers as set forth in claim 3, which comprises admixing dopant to the gas prior to a selected one of said cooling steps, whereby a correspondingly doped crystal layer is grown during said cooling step.

6. The method of producing crystalline semiconductor wafers with epitaxial layers, as set forth in claim 3, which comprises admixing respectively different dopants to the gas prior to respective sequential cooling steps, whereby p-n junction layers are grown.

7. The method ofjpr'oducing gallium-phosphide semiconductor crystals, which comprises heating gallium and phosphorus together with halogen in an evacuated vessel and thereby producing a gallium-phosphide-halogen vapor therein, thereafter subjecting a zone of the vessel to stepwise cooling, at a rate of '8 to 50C. per minute, and intermediate temperature holding steps whereby respective crystal layers are deposited upon each other during the respective cooling-step periods, and admixing vapor ous dopant to the vapor prior to a selected one of said cooling steps, whereby a substrate is grown and epitaxial doped layers are bilaterally grown on the substrate during respective consecutive cooling steps.

8. The method of producing gallium-arsenide semiconductor crystals, which comprises heating gallium and arsenic together with halogen in an evacuated vessel and thereby producing a gallium-arsenide-halogen vapor therein, thereafter subjecting a zone of the vessel to stepwise cooling, at a rate of 8 to 50 C. per minute, and intermediate temperature holding steps whereby respective crystal layers are deposited upon each other during the respective cooling-step periods, and admixing vaporous dopant to the vapor prior to a selected one of said cooling steps, whereby a substrate is grown and epitaxial doped layers are bilaterally grown on the substrate during respective consecutivecooling steps.

(References on following page) 0? 10 c/ References Cited by the Examiner International Conference on Crystal Growth held at UNIT D A E P A E Cooperstown, N.Y., August 2729, 1958, pages 49-54.

E ST T S T NTS Loonam: Principles and Applications of the Iodide 2763581 9/56 Freedman 148-175 Process, Journal of the Electrochemical Society, March ,813,811 11/57 Sears 1481.6 5 1959 23 244 2842468 7/58 Brenner 148 1'6 Minden: Letter entitled Leaves of GaAs, Journal of OTHER REFERENCES Applied Physics, vol. 33, pages 243-244.

Antel et 211.: Article in Journal of the Electrochemical Vapor Makes Galiium phosphide Chem Society, VOL 106, June 1959, pages ical and Engineering News, April 18, 1960, page 72.

Growth and Perfection of Crystals, Proceedings of an 10 DAVID L. RECK, Primary Examiner. 

1. THE METHOD OF PRODUCING MONOCRYSTALLINE WAFERS BY PRECIPITATING SUBSTANCE FROM THE HEATED GASEOUS PHASE IN A COOLING ZONE OF AN EVACUATED VESSEL, WHICH COMPRISES HEATING THE VESSEL AND THE SUBSTANCE CONTAINED THEREIN TO ABOVE THE VAPORIZATION TEMPERATURE OF THE SUBSTANCE, AND THEREAFTER SUBJECTING THE COOLING ZONE OF THE VESSEL TO COOLING AT A RATE OF 8 TO 50*C. PER MINUTE IN A PLURALITY OF STEPS WITHIN THE INCANDESCENT RANGE, AND MAINTAINING THE TEMPERATURE APPROXIMATELY CONSTANT BETWEEN THE RESPECTIVE COOLING STEPS FOR AN AVERAGE INTERVAL OF TIME APPROXIMATELY CORRESPONDING TO THAT OF THE COOLING STEPS. 